The partial pressure or percentage of oxygen (and carbon dioxide) are particularly useful in order to satisfy the needs of the user while reducing the consumption in pure oxygen (provided by an oxygen cylinder, a chemical generator or a liquid oxygen converter) or gas highly enriched in oxygen provided in particular by an on-board oxygen generator system (OBOGS).
But, when two gases having different mixtures successively flow in opposite directions in a chamber, the measurement of a characteristic of a gaseous constituent in the first gas mixture flow is disturbed by the second gas mixture. The invention aims at reducing this problem.